+Known issues and warnings
+.........................
+
+The most time consuming task for building the documentation is running
+LilyPond to build images of music, and there cannot be several
+simultaneously running `lilypond-book' instances, so `-j' `make' option
+does not significantly speed up the build process. To help speed it
+up, the makefile variable CPU_COUNT may be set in `local.make' or on
+the command line to the number of `.ly' files that LilyPond should
+process simultaneously, e.g. on a bi-processor or dual core machine
+
+ make -j3 CPU_COUNT=3 doc
+
+The recommended value of CPU_COUNT is one plus the number of cores or
+processors, but it is advisable to set it to a smaller value if your
+system has not enough RAM to run that many simultaneous LilyPond
+instances.
+
+ If source files have changed since last documentation build, output
+files that need to be rebuilt are normally rebuilt, even if you do not
+run `make doc-clean' first. However, building dependencies in the
+documentation are so complex that rebuilding of some targets may not be
+triggered as they should be; a workaround is to force rebuilding by
+touching appropriate files, e.g.
+
+ touch Documentation/user/*.itely
+ touch input/lsr/*.ly
+
+Building documentation without compiling LilyPond
+.................................................
+
+The documentation can be built locally without compiling LilyPond
+binary, if LilyPond is already installed on your system.
+
+ From a fresh Git checkout, do
+
+ ./autogen.sh # ignore any warning messages
+ cp GNUmakefile.in GNUmakefile
+ make -C python
+ nice make LILYPOND_EXTERNAL_BINARY=/path/to/bin/lilypond doc
+
+ Please note that this may break sometimes - for example, if a new
+feature is added with a test file in input/regression, even the latest
+development release of LilyPond will fail to build the docs.